If the rumors are accurate, there won’t be any major external changes. This would be the classic speed bump with new processors (Intel’s 7th-generation Core chips) and other possible under-the-hood improvements, which isn’t surprising when the current design isn’t even a year old. While you’re unlikely to see a dramatic leap in performance, this could lead to longer battery life and better integrated graphics performance (mainly useful for 13-inch models). You could also see the MacBook Pro support up to 32GB of RAM, addressing a pain point for pros who struggle with the current 16GB limit.
However, the very fact that there’s an update so relatively soon is notable by itself. It’s not necessarily proof that Apple has renewed its commitment to the Mac (it may just be easier to update now that Apple has the support it needs for Thunderbolt 3 and USB-C). However, it definitely suggests that the days of agonizingly long update intervals could be over. As MacRumors notes, there was a year and a half between the old MacBook Pro and the current model — if a refresh arrives at WWDC, that gap will have shrunk to just 220 days. While we wouldn’t expect updates to always show up that quickly (that’s partly dependent on Intel), Apple may well be back to keeping pace with the latest CPU technology.